https://news.ycombinator.com/newsguidelines.html

Because way more people have opinions about e.g. asteroid game scripts on web pages than have opinions on RCUs, these subthreads spread like kudzu.

This is done by expand_fdtable() in the kernel. It contains the following code:

        if (atomic_read(&files->count) > 1)
          synchronize_rcu();

If the fdtable is expanded prior to creating a new thread, as the test program optionally will do by calling dup(0, 666) if supplied a command line argument, this avoids the synchronize_rcu() call because at this point files->count == 1. Therefore, if this is done, there will be no delay later on when creating all the sockets as the fdtable will have sufficient capacity.

By contrast, the OpenBSD kernel doesn't have anything like RCU and just uses a rwlock when the file descriptor table of the process is being modified, avoiding the long delay during expansion that may be observed in Linux.

My understanding is that using tsc directly is tricky. The rate might not be constant, and the rate differs across cores. [1]

[1]: https://www.pingcap.com/blog/how-we-trace-a-kv-database-with...